I'm not sure about that. There have been plenty of mistakes made by astro/cosmonaughts over the years. They are human. If "any mistake" is lethal, then we should have seen many more deaths.
I wrote about cosmonaut's training, not a trained cosmonaut's job. Those who got hurt or died in training didn't get to fly rockets. Also, that was in the Gagarin's time; after getting enough data they probably relaxed the training a bit in the next decades.
